HER2 is a 185-kDa cell surface receptor tyrosine kinase and member of the epidermal growth factor receptor (EGFR) family that comprises four distinct receptors: EGFR/ErbB-1, HER2/ErbB-2, HER3/ErbB-3, and HER4/ErbB-4. Both homo- and heterodimers are formed by the four members of the EGFR family, with HER2 being the preferred and most potent dimerization partner for other ErbB receptors (Graus-Porta et al., Embo J 1997; 16:1647-1655; Tao et al., J Cell Sci 2008; 121:3207-3217). HER2 can be activated by overexpression or by heterodimerization with other ErbBs that can be activated by ligand binding (Riese and Stern, Bioessays 1998; 20:41-48). For HER2, no ligand has been identified. HER2 activation leads to receptor phosphorylation, which triggers a cascade of downstream signals through multiple signaling pathways, such as MAPK, phosphoinositol 3-kinase/AKT, JAK/STAT and PKC, which ultimately results in the regulation of multiple cellular functions, such as growth, survival and differentiation (Huang et al., Expert Opin Biol Ther 2009; 9:97-110).
Much of the attention on HER2 in tumors has been focused on its role in breast cancer, in which HER2 overexpression is reported in approximately 20% of the cases and is correlated with poor prognosis (Reese et al., Stem Cells 1997; 15:1-8; Andrechek et al., Proc Natl Acad Sci USA 2000; 97:3444-3449; and Slamon et al., Science 1987; 235:177-182). Besides breast cancer, HER2 expression has also been associated with other human carcinoma types, including prostate cancer, non-small cell lung cancer, bladder cancer, ovarian cancer, gastric cancer, colon cancer, esophageal cancer and squamous cell carcinoma of the head & neck (Garcia de Palazzo et al., Int J Biol Markers 1993; 8:233-239; Ross et al., Oncologist 2003; 8:307-325; Osman et al., J Urol 2005; 174:2174-2177; Kapitanovic et al., Gastroenterology 1997; 112:1103-1113; Turken et al., Neoplasma 2003; 50:257-261; and Oshima et al., Int J Biol Markers 2001; 16:250-254).
Trastuzumab (Herceptin®) is a recombinant, humanized monoclonal antibody directed against domain IV of the HER2 protein, thereby blocking ligand-independent HER2 homodimerization, and to a lesser extend heterodimerization of HER2 with other family members in cells with high HER2 overexpression (Cho et al., Nature 2003; 421:756-760 and Wehrman et al., Proc Natl Acad Sci USA 2006; 103:19063-19068). In cells with modest HER2 expressing levels, trastuzumab was found to inhibit the formation of HER2/EGFR heterodimers (Wehrman et al., (2006), supra; Schmitz et al., Exp Cell Res 2009; 315:659-670). Trastuzumab mediates antibody-dependent cellular cytotoxicity (ADCC) and prevents ectodomain shedding, which would otherwise result in the formation of a truncated constitutively active protein in HER2 overexpressing cells. Also inhibition of both in vitro and in vivo proliferation of tumor cells expressing high levels of HER2 has been reported for trastuzumab (reviewed in Nahta and Esteva, Oncogene 2007; 26:3637-3643). Herceptin® has been approved both for first-line and adjuvant treatment of HER2 overexpressing metastatic breast cancer, either in combination with chemotherapy, or as a single agent following one or more chemotherapy regimens. Trastuzumab has been found to be effective only in 20-50% of HER2 overexpressing breast tumor patients and many of the initial responders show relapse after a few months (Dinh et al., Clin Adv Hematol Oncol 2007; 5:707-717).
Pertuzumab (Omnitarg™) is another humanized monoclonal antibody. It is directed against domain II of the HER2 protein, resulting in inhibition of ligand-induced heterodimerization (i.e., HER2 dimerizing with another member of the ErbB family to which a ligand has bound); a mechanism reported to not strictly require high HER2 expression levels (Franklin et al., Cancer Cell 2004; 5:317-328.). Although pertuzumab also mediates ADCC, the main mechanism of action of pertuzumab relies on its dimerization blockade (Hughes et al., Mol Cancer Ther 2009; 8:1885-1892). Moreover, pertuzumab was found to enhance EGFR internalization and downregulation by inhibiting the formation of EGFR/HER2 heterodimers, which otherwise tethers EGFR at the plasma membrane (Hughes et al., 2009, supra). This correlates with the observation that EGFR homodimers internalize more efficient than EGFR/HER2 dimers (Pedersen et al., Mol Cancer Res 2009; 7:275-284. The complementary mechanisms of action of pertuzumab and trastuzumab reportedly results in enhanced anti-tumor effects and efficacy when combined in patients who progressed during prior trastuzumab therapy (Baselga et al., J Clin Oncol 2010; 28:1138-1144), and a phase III trial to evaluate this antibody combination together with Docetaxel in previously untreated HER2-positive metastatic breast cancer is underway.
An alternative approach to improve targeted antibody therapy is by delivering cytotoxic cells or drugs specifically to the antigen-expressing cancer cells. For example, the so-called trifunctional antibodies are bispecific antibodies, targeting with one arm the antigen on the tumor cell and with the other arm for instance CD3 on T cells. Upon binding, a complex of T cells, tumor cells and effector cells that bind Fc is formed, leading to killing of the tumor cells (Muller and Kontermann, BioDrugs 2010; 24:89-98.). Ertumaxomab is one such trifunctional antibody against HER2, which induces cytotoxicity in cell lines with low HER2 expression and which is in Phase II clinical development in metastatic breast cancer (Jones et al., Lancet Oncol 2009; 10:1179-1187 and Kiewe et al., Clin Cancer Res 2006; 12:3085-3091).
A HER2 antibody drug conjugate (ADC) is currently in clinical development. T-DM1 consists of trastuzumab conjugated to the fungal toxin maytansine. In Phase II trials, responses in a heavily pretreated patient cohort including prior trastuzumab and/or lapatinib therapy were reported Burris et al, 2011, J Clin Oncol 29: 398-405 and Lewis Phillips et al., Cancer Res 2008; 68:9280-9290). Preliminary data from a Phase II trial determining efficacy and safety of T-DM1 versus trastuzumab plus docetaxel in her2-positive metastatic breast cancer patients with no rior chemotherapy for metastatic disease were reported (Perez et al, Abstract BA3, European Society for Medical Oncology meeting 2010). A Phase III trial to evaluate T-DM1 efficacy and safety versus capecitabine+lapatinib in patients with HER2-positive locally advanced or metastatic breast cancer who received prior trastuzumab therapy is ongoing.
While many factors are involved in selecting a suitable antibody for HER2 targeted therapy, it is typically an advantage for an ADC approach if the HER2-antibody complex efficiently internalizes upon antibody binding. Studies on murine HER2 antibodies have shown that certain combinations of antibodies instigate HER2 endocytosis (Ben-Kasus et al., PNAS 2009; 106:3294-9). Human HER2 antibodies F5 and C1 have been reported to internalize relatively rapidly on their own and to bind the same epitope (WO 99/55367 and WO 2006/116107). As compared to EGFR, however, internalization of HER2 is impaired. Indeed, EGFR homodimers internalize much more efficiently than HER2 homodimers (Dinh et al., Clin Adv Hematol Oncol 2007; 5:707-717). EGFR, and also HER3, can increase endocytosis of HER2 by the formation of EGFR/HER2 and HER3/HER2 heterodimers, respectively (Baulida et al., J Biol Chem 1996; 271:5251-5257; Pedersen N M, et al., Mol Cancer Res 2009; 7:275-84).
The complex mechanisms regulating the function of HER2 warrant further research on new and optimized therapeutic strategies against this proto-oncogene. Accordingly, there remains a need for effective and safe products for treating HER2-related diseases, such as cancer.